One device for measuring ionic activity in liquids is described in commonly assigned U.S. Pat. No. 4,184,936 (issued Jan. 22, 1980 to Paul et al). This device includes two ion-selective, solid electrodes mounted on a frame and a capillary bridge which promotes ionic migration between the two electrodes. The capillary bridge includes a nonporous support layer, a nonporous top layer and a porous layer therebetween. A pair of holes through the bridge and aligned with the electrodes allow liquid access to both electrodes. When a drop of reference solution of known ionic activity is applied to one fluid access hole and a drop of test solution of unknown ionic activity is applied to the other fluid access hole, the drops spread into the porous layer until contact is made at a thin junction interface, permitting ionic migration between the drops. The time it takes for the drops to spread adequately to form the thin junction interface and to allow ionic migration is termed the "junction time." An electrometer can be used to measure the electrical potential at the interfaces between each solution drop and its associated electrode to provide an indication of ionic activity in the test solution.
In commonly assigned U.S. Pat. No. 4,336,091 (issued June 22, 1982 to Gottermeier), an improved analyte activity measuring device and method of making same are described. The described device comprises two electrodes held in a frame by an integral cover sheet and capillary bridge. The cover sheet is formed of a nonporous material with an encapsulated porous ribbon, and fluid access holes extend through the cover sheet in alignment with the porous ribbon and each electrode. The test and reference fluids are confined to free spaces within the porous ribbon to form an ion junction between the electrodes.
The method of making the devices of the Gottermeier patent allows for improved manufacturing efficiencies in several respects. In this manufacturing method, however, the porous ribbon through which test and reference liquids travel is highly compressed in the encapsulation step. Typically, this porous ribbon is a porous paper composed of cellulosic fibers. When the paper is wetted with liquid samples, the cellulosic fibers swell, thereby reducing the free space in the ribbon even more. As a result, travel of the liquid samples within the ribbon is severely restricted and junction time is long.
Long junction times are highly undesirable in the health care field for both medical and economic reasons. Health care practitioners desire rapid analyses of biological fluids in an effort to render prompt medical diagnosis. Health care administrators desire to use highly automated analytical equipment at the most efficient rates of operation. Differences in junction time of only a few seconds, multiplied by the thousands of analytical tests made daily, can frustrate both desires.